RF filters are commonly used in RF communications circuits to remove signals at unwanted RF frequencies. An RF receiver may use RF filters to remove unwanted signals that have been received through an antenna, or signals from other circuitry, such as an RF transmitter. An RF transmitter may use RF filters to remove noise from a transmitted signal. A common type of filter circuit is a surface acoustic wave (SAW) filter, which may be used as a bandpass filter. One example of a bandpass filter 10 is shown in FIG. 1. A filter input FIN feeds the bandpass filter 10, which filters out high frequency signals and low frequency signals. A filter output FOUT is provided from the bandpass filter 10. FIG. 2A is a graph showing a constant amplitude signal at all frequencies being applied to the filter input FIN. FIG. 2B shows the response of the bandpass filter 10 at the filter output FOUT. Frequencies below a high pass filter break frequency FHP are filtered out. Frequencies above a low pass filter break frequency FLP, are filtered out. Frequencies between the two break frequencies FHP, FLP, called the pass band, are allowed to pass through the bandpass filter 10. A center frequency FC is the average of the high pass filter break frequency FHP and the low pass filter break frequency FLP.
RF bandpass filters are typically limited in their roll-off characteristics. It is difficult to produce an RF bandpass filter with a high enough Q to adequately filter out all of the unwanted signals and noise. Q indicates the effectiveness of an RF filter. This is one of the reasons that super-heterodyne receivers are used. In a super-heterodyne receiver, a received RF signal is mixed with an RF local oscillator signal to create an intermediate frequency (IF) signal, the frequency of which is a difference between the frequency of the received RF signal and the frequency of an RF local oscillator signal. Since the IF signal is at a lower frequency, any interfering signals may be filtered out more effectively in the IF section of the receiver than in the RF section. It may be easier to construct a high Q IF filter than a high Q RF filter. Since, by definition, the frequency of the RF local oscillator signal is the center frequency of a desired received RF signal, there is a need for an RF filter that is closely aligned with the frequency of the RF local oscillator signal. As a result, RF bandpass filters may have to be tuned to the desired received center frequency and must be stable in the presence of environmental changes, such as temperature. Thus, there is a need for an RF filter having the high Q roll-off characteristics of an IF filter, which is aligned with the desired received center frequency resulting in improved filtering characteristics.